Plant extracts as antimicrobials in ruminants
Microbial fermentation in the rumen dominates the nutrition of ruminant animals. Microbial ecosystems lend themselves to manipulation by external means, and it has long been clear that feed additives could be used to improve the nutrition of ruminants by manipulating ruminal fermentation. Ionophores and antibiotics have been used in the past to achieve some nutritional goals, however the recent ban of antimicrobial feed additives in the EU has lead to renewed interest in plants or their extracts as feed additives, with some success. This paper reviews recent progress in understanding how plants and their extracts may be used as rumen manipulating agents, with some benefits that were provided by ionophores and others that may improve the health of the animal and the healthiness of ruminant products.
Targets for manipulation
Breakdown of dietary protein
Dietary protein entering the rumen is broken down in an apparently uncontrolled way, resulting in ammonia formation and subsequent loss of N in the urine. The low efficiency of nitrogen retention which results represents a major economic loss, causes metabolic stress in the animal, and also places a burden on the environment, in the form of nitrogen-rich wastes.
Breakdown of microbial protein
Protozoa consume large quantities of bacteria in the rumen. The protein breakdown which results causes the net yield of microbial protein resulting from rumen fermentation to be decreased by up to 50%. If the protozoa could be suppressed, there would be less ammonia formation and less need for dietary protein supplementation.
Methane formation
Methane is a greenhouse gas many times more potent than CO2. Its concentration in the atmosphere has doubled over the last century. Ruminants are major contributors to biogenic methane formation, and it has been estimated that preventing methane formation from ruminants would stabilise atmospheric methane concentrations. Decreasing methane emissions would also lead to improved energy retention in the animal.
Digestive disorders
Bloat is a disorder in which the gases formed by fermentation are prevented from escaping because a stable foam forms in the rumen. Lactic acidosis occurs when a rapidly degraded feed is introduced too quickly, or when concentrates form a high proportion of the diet, volatile fatty acid production exceeds the buffering capacity of the rumen, rumen pH falls, and only lactic acid-producing bacteria can grow.
Fatty acid biohydrogenation
Dairy products and milk are generally considered to be unhealthy, because they are high in saturated fats. The aim of manipulation is to increase the content of health-promoting unsaturated fatty acids, particularly conjugated linoleic acids (CLA), in ruminant products by controlling biohydrogenation of unsaturated fatty acids in the rumen.
Searching for plants/extracts with a particular function
One strategy is to focus on a target activity and screen a large number of samples for their influence on that activity. Projects of this nature are springing up in several countries. Three in which the Rowett group have been involved will be described. Investigation, in collaboration with the International Livestock Research Institute, Addis Ababa, of the effects of different possible 'multipurpose trees' on protozoal activity lead to the opportunity to use the foliage from some trees, including Sesbania sesban and Enterolobium cyclocarpum, to
suppress protozoal activity and thereby to improve microbial protein flow from the rumen (Teferedegne et al, 1999). The active agent was subsequently found to be saponins.
Rumenup (QLK5-CT-2001-00992) 2001-2005 : New plants and plant extracts to decrease methane and nitrogenous emissions from ruminants and to alleviate nutritional stress was a Framework 5 project, whose aim was to develop new plants or plant extracts as dietary supplements for ruminants to replace chemical additives and growth-promoting antibiotics. Five hundred plant materials were collected from botanical and industrial collections on the basis of known secondary metabolites and/or traditional uses in, for example, herbal medicine. All samples were screened in vitro for their effectiveness in inhibiting rumen ciliate protozoa, rumen proteolysis, methane formation, microbial protein synthesis, lactic acidosis and bloat. The samples were also investigated to ensure that potentially useful samples had no detrimental effect on the other basic functions of the fermentation, such as fibre digestion and volatile fatty acid production.
A total of 23 samples was identified to have potential for development as feed additives which could manipulate fermentation in one or more of the target areas without having detrimental effects on overall fermentation (Becker et al, 2005). Several plants appeared to hold particular promise, namely Knautia arvensis (field scabious) in suppressing ruminal proteolysis, Lonicera japonica (Japanese honeysuckle), Gentiana asclepidea (willow gentian) and Bellis perennis (daisy) in inhibiting the activity of rumen ciliate protozoa, and Urtica dioica (stinging nettles) and Lactuca sativa (lettuce) in stabilising ruminal pH. The activity of the antiprotozoal plants was almost certainly due to their saponins. However, the active component of the others is not known. The antiproteolytic activity of K. arvensis was noteworthy, because unlike many other samples that were inhibitory to proteolysis, it contained no tannins, and its effect was adaptive rather than acute.
A Framework 6 project is using the same resource in terms of (recollected) plant materials and the knowledge gained in Rumen-up to broaden the animal species range and to explore other applications in ruminants. This project is called REPLACE (Food-CT-2004-506487) : Plants and their extracts and other natural alternatives to antimicrobials in feeds. The properties being targeted in ruminants include control of parasites, fatty acid biohydrogenation and forage digestion. Chrysanthemum coronarium has been identified as a candidate additive to improve milk fatty acid composition.
Starting with the plant extract: what does it do?
In several cases, our approach has been to take an extract already in common use and to find out what it does to ruminal fermentation and ruminal microorganisms. Historically, this is how ionophores were investigated. Nutritionists used the ionophores before the mode of action in manipulating ruminal fermentation was known. The same approach has been taken with a few plant extracts (Wallace, 2004). Saponins, investigated mainly as sarsaponin, suppress the bacteriolytic activity of rumen ciliate protozoa and thereby enhance total microbial protein flow from the rumen (Cheeke, 1998). The effects of some saponins seems to be transient, which may stem from the hydrolysis of saponins to their corresponding sapogenin aglycones, which are much less toxic to protozoa. Saponins also have selective antibacterial effects which may prove useful in, for example, controlling starch digestion (Wang et al. 2000). Essential oils cause rates of NH3 production from amino acids in ruminal fluid taken from sheep and cattle receiving the oils to decrease, yet proteinase and peptidase activities were unchanged (McIntosh et al, 2003). Hyper-ammonia-producing bacteria were the most sensitive of ruminal bacteria to essential oils in pure culture. Essential oils also slowed colonisation and digestion of some feedstuffs. Ruminobacter amylophilus may be a key organism in mediating these effects. Busquet et al. (2005) noted that garlic oil inhibited methane formation. Tannins bind to protein and may be used to slow the degradation of those proteins which are degraded too rapidly in the rumen (Waghorn and McNabb, 2003).
Conclusion
The plant kingdom contains an almost unlimited number of plants and chemical compounds that could be used to manipulate undesirable aspects of ruminal fermentation and to promote the more desirable activities. Well known classes of chemicals, such as saponins, essential oils and tannins, have already been investigated in some detail, but others which are less well known or not identified to date may be useful. Nevertheless, a major obstacle to commercialization in the EU is the hurdles to be surmounted in the regulatory process. This is a major disincentive to truly innovative uses for plants or their extracts in ruminant feeding.
by R.J. Wallace - Rowett Research Institute
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